The present invention relates to coating compositions which increase the release force of pressure sensitive adhesives (PSA's) adhered thereto. The present invention further relates to a method of release control by crosslink density (or bulk modulus) control.
In the field of pressure sensitive release materials a perennial problem has been achieving elevated release forces (tighter release) coupled with smooth nonrapsy release. Current technology uses an R.sub.3 SiO.sub.1/2 or SiO.sub.2 (wherein R is a monovalent hydrocarbon radical) resin to give elevated release. The problem with this approach is that the profile of release against delaminating speed is often totally unsatisfactory with release forces being lower at high speeds than at low. This leads to converting problems with small labels or high speed converting. This problem is especially pertinent to 100% solids materials.
There are instances when there is a need in the paper coatings industry for a composition which provides increased release values at high delamination speeds. For example, in a process for preparing pressure sensitive labels a laminate of adhesive-bearing label stock and silicone-bearing backing paper is die cut on its label stock side to provide a plurality of labels surrounded by a waste matrix, both still adhered to the backing paper. The waste matrix is typically rapidly stripped away and discarded, leaving only the die-cut labels adhered to the uncut backing paper. For such a process it is highly desirable that the force required to separate the adhesive from the coating be sufficiently low to allow easy stripping of the waste matrix but sufficiently high to retain the labels on the backing paper during the stripping operation. Another example of when high release force is required in the paper coatings industry is in the preparation of rolls of pressure sensitive adhesive tape a support material, such as paper or polymeric film, is provided with a cured silicone release coating and a pressure sensitive adhesive coating. The resulting stock is rolled into a roll configuration so that the adhesive coating is adhered to the silicone release coating over substantially its entire surface. In such a product the force required to separate the adhesive from the coating should be sufficiently low to allow the easy unwinding of the roll of tape, when desired, but sufficiently high to retain the tape in the roll configuration during the preparing, storing, and handling of the roll. Compositions which control the release force of silicone paper release coatings have been described in the art. For example, Mestetsky, U.S. Pat. No. 3,855,052, teaches a controlled release coating composition comprising an organic solvent solution of an organopolysiloxane and a solvent soluble polymer of a polymerizable unsaturated aliphatic dibasic acid or derivative thereof. Mestetsky also teaches that incremental changes in the proportion of said solvent soluble polymer lead to incremental changes in the release force of the coatings produced with such compositions. A typical composition of the '052 patent consists of toluene, polydimethylsiloxane, and a copolymer of maleic anhydride and alkylvinyl ether.
Traver et al., U.S. Pat. No. 4,190,688 teaches a paper release coating emulsion composition comprising a vinyl-containing polymer, a hydride crosslinking agent, water, and an emulsifying agent which when applied to kraft paper gave release paper with good release properties.
Homan et al., U.S. Pat. No. 4,525,566, discloses moisture-curable silicone compositions for adhesive release coatings comprising an alkoxysilylorganosiloxane, a titanium ester, and an aminoorganosiloxane having an adhesive release force, when cured, which is directly related to the amount and type of aminoorganosiloxane. It is further disclosed that the compositions are useful for coating a flexible sheet material, such as paper or polymeric film, to provide a surface that will release pressure sensitive adhesive with a pre-selected release force of up to 400 g/in (154 N/m).
In unexamined published Japanese patent application No. (JP-A-No.) 2113015, it is disclosed that polyurethane resin release agents produced by reacting a polyol component containing at least partially copolymers comprising active hydrogen containing siloxane compounds and caprolactones, a polyisocyanate component, and chain extenders, are useful for rendering of releasability to the back faces of adhesive tapes and adhesive sheets, and the surface of release paper.
Hulme et. al., PCT Application No. 8912549, teaches a release liner for use with pressure sensitive adhesives comprising a liner substrate having a release coating derived from a blend of an aqueous emulsion of a curable vinyl addition silicone polymer and a particle component which is preferably a resin. The polymer system comprises vinyl silicone polymer and silicone hydride crosslinker, catalyzed by a group VIII metal catalyst, preferably platinum. Hulme et al. further teaches that the release force or peel at a variety of peel rates can be controlled by the silicon particle ratio, the nature of the particles used, the degree of interaction of the two, and the crosslink density of the cured silicone and coating weight.
Coating compositions having vinyl-endblocked polydiorganosiloxanes have been described in the art. Hockemeyer et al. in U.S. Pat. No. 4,154,714 teaches coating compositions which impart adhesive repellent or non-adherent properties to substrates coated therewith comprising (1) vinyl endblocked diorganopolysiloxanes having an average viscosity in excess of 10.sup.6 cP at 25.degree. C. in which from 3 to 39 percent of the nonterminal siloxane units are diphenylsiloxane units and at least 50 mole percent of the remaining organic radicals on the siloxane units are methyl radicals, (2) organopolysiloxanes having at least 3 Si-bonded hydrogen atoms per molecule, (3) catalysts which promote the addition of Si-bonded hydrogen atoms to the vinyl groups, and (4) an inert organic solvent. Also Jeram et al. in U.S. Pat. No. 4,340,709 discloses an addition curing silicone composition with a low viscosity in the uncured state and high physical strength in the uncured state comprising a vinyl-containing diorganopolysiloxane polymer, a platinum catalyst, a hydride crosslinking agent and a linear hydride coupler in which the viscosity of the linear hydride coupler varies from 1 to 500 centipoise at 25.degree. C.
Alkenyl polydiorganosiloxanes have also been taught. Okami et al., in European Patent Application No. EPO 409229 teaches an addition type curable silicone rubber composition comprising (a) 100 parts by weight organopolysiloxanes containing 2 or more alkenyl groups bonded to Si atoms in one molecule, (b) 3 to 70 parts by weight of organopolysiloxanes containing one alkenyl group bonded to Si atom in one molecule on average, (c) organohydrogenpolysiloxanes containing two or more H bonded to Si atoms in one molecule in an amount providing 0.5 to 5 H bonded to Si atoms per 1 alkenyl group of (a) and (b), and (d) a catalytic amount of a platinum compound.
Other release coatings described in the art which employ compositions containing olefinic hydrocarbon radicals, silicon-bonded hydrogen atoms, and inhibitors in a metal catalyzed reaction are taught in U.S. Pat. Nos. 4,562,096, 4,609,574, 4,774,111, and 5,036,117. However none of these references disclose a composition which includes olefinic hydrocarbon-endblocked and silicon-bonded hydrogen-endblocked polysiloxanes in combination to increase the release force of silicone paper release coatings at high delaminating speeds.
As stated hereinabove, there is a need for improved silicone release coatings wherein their adhesive-release forces can be increased when desired and also maintain release stability. The present invention is therefore directed to the use of siloxane polymers to increase the release force of solventless paper release coatings while maintaining stable release control.